The invention relates to a device for crystallizing a component from a liquid mixture that contains it, and its use particularly for separating paraxylene from a hydrocarbon mixture that contains aromatic isomers with 8 carbon atoms.
The invention also applies very particularly to the purification to 99.9%, for example, of paraxylene for the preparation of terephthalic acid, an intermediate petrochemical for the synthesis of textile fibers.
Among the previous processes that are suitable for the purification of paraxylene, crystallization is the one that has been used most, even though it is limited by a low recovery level due to the existence of a eutectic whose paraxylene concentration in the mixture reaches about 10 to 13%.
With the development of techniques for separation by adsorption in a molecular sieve, it is possible to achieve an excellent paraxylene yield, greater than, for example, 98%, independently of the eutectic composition limitation.
Usually, a recovery in excess of 98% of paraxylene is obtained with the processes of a simulated countercurrent fluid bed (U.S. Pat. No. 2,985,589) when the purity of the product is close to 95% by weight A higher purity, exceeding 99%, however, can be reached at the expense of the yield.
Since an adsorption process makes it possible to carry out the separation of paraxylene with a high yield at the expense of purity while a crystallization process makes it possible to obtain a more pure product to the detriment of the yield, the applicant has proposed a hybrid process that combines adsorption in a molecular sieve of aromatic C8 isomers, followed by crystallization of the paraxylene-enriched fraction (U.S. Pat. No. 5,401,476, and EP-B.531191 that are incorporated as references).
This process thus combines the advantages of a high yield and a very high purity of the wanted product compared to the processes of adsorption or of crystallization used separately.
Furthermore, the technology of crystallization is very old and has hardly been updated, considering the industrial breakthrough of the adsorption process in the simulated fluid bed.
The technological background is illustrated by the following documents: EP-A 611 589, EP-A 455 243, DE-A 2 222 755 and DE-C 972 036.
Crystallization for separating the paraxylene from a mixture of xylenes is generally carried out at very low temperatures, located in the range of those that can be attained by refrigeration with ethane or with ethylene. The costs of refrigeration and the consumption of energy are high, particularly because it is necessary to produce a cascade of cold cycles, with intermediate refrigeration with propane or with propene.
Certainly, this consumption is reduced when the operation is carried out in a higher range of crystallization temperatures, +10 to xe2x88x9230xc2x0 C., for example, as is the case when the crystallization feedstock is enriched to more than 50%, for example, preferably with more than 70% paraxylene, by an enrichment process by adsorption of xylenes or by paraselective dismutation of toluene, for example.
Moreover, the process of continuous crystallization by indirect exchange generally requires that the exchangers be scraped, which is an operation that is delicate and energy-intensive, regardless of the selected cooling level.
One of the objects of the invention is to remedy the drawbacks that are mentioned above.
Another object is to propose a crystallization technique by direct exchange of kilogram calories with a feedstock which is simpler and easier to use.
Another object is the use of a process of crystallization and a device that correspond, at any desired level of cooling temperature and at very much the same operating expense, knowing, of course, that the power of the gas compressors will depend on the desired cooling temperature.
It has therefore been observed that it was possible to crystallize a product in a liquid mixture that contains it, constituting the crystallization feedstock, by direct exchange of cold with a cold cryogenic gas that is obtained by approximately isentropic expansion of this gas, under very favorable and very economical conditions.
In particular, the scope of the present invention is not limited to any particular geometry of a crystallizer since the present invention provides the broad concept of any type of crystallizer combined with a turbo-expander which can cool the fluid obtained from the crystallizer for purposes of recycling it to the crystallizer.
Thus, there is provided a device for crystallization of a component from a liquid mixture comprising a chamber, a first conduit connected to said chamber for supplying said mixture, a second conduit connected to and in combination with said chamber for introducing cold gas, a third conduit for withdrawing warmed gas from said crystallizer, a turbo-expander for cooling gas withdrawn from said chamber, and a fourth conduit leading from the turbo-expander to said chamber.
Described even more specifically is a device for crystallization of a component from a liquid mixture that contains it and that comprises an elongated chamber that has in its upper part means for supplying said mixture, collecting means, and in its lower part, crystals of the component in suspension in a mother liquor, connected to means for separation and for purification of the crystals that are obtained, means for supplying a cold gaseous fluid that is introduced at at least one point in the lower part of the chamber and means for drawing off the so-called hot gaseous fluid that is placed at the upper part of the chamber, resulting from the direct countercurrent heat exchange of the cold gaseous fluid with the mixture, whereby said device also comprises means for suitable shaping of a descending flow of the mixture such that the cold gaseous fluid that circulates upward exchanges directly from the cold with the mixture that is shaped, whereby the means for drawing off the hot gaseous fluid are connected to at least one fluid recompression means (24b), whereby the recompression means has an output that is connected to at least one heat exchanger, whereby the heat exchanger has an output that is connected to a turbo-expander (30), and whereby the turbo-expander has an output that is connected to cold gaseous fluid supply means (31).
According to an embodiment of the device, the means for suitable shaping of the descending flow of the mixture comprise at least one sprayer that is suitable for forming droplets of the mixture measuring between 50 and 500 micrometers and preferably according to the distribution of sizes of 150 to 200 micrometers for at least 80% of the droplets.
According to another embodiment of the device, the chamber can contain a large number of filaments that are suspended, non-contiguous, and approximately parallel to its longitudinal axis, supported by attachment means that are placed in the vicinity of, for example, the upper end of the chamber and that occupy approximately the entire section of the chamber.
The sprayer that is mentioned above shapes the droplets of the mixture that constitutes the feedstock, which can be deposited on the filaments in the form of a thin film which is subjected to crystallization by direct contact with the cold gas that rises into the crystallization chamber.
These vertical filaments can be hollow, tubes, for example, to lighten the device and/or to make possible the circulation of a hot fluid that reheats that wall of the filament to promote the presence of a liquid film on the latter and the downtake of the crystals into the lower part of the crystallization chamber.
This hot fluid can be a portion of the gaseous fluid that is recovered from the crystallization chamber and recompressed.
According to a variant, the flowing of the liquid mixture onto the filaments can be carried out differently.
Actually, the means for suitable shaping of the flow comprise a chamber for communication with said means for supplying the mixture that comprise a large number of flow orifices that are pierced on the lower transverse wall of the chamber and are equal in number to that of the filaments, whereby each filament passes through an orifice whose surface area is larger than that of the section of the filament, whereby said chamber contains a volume of mixture such that the height of the mixture, in combination with the available flow surface between the orifice and the filament, makes it possible to wet the filament and to ensure a suitable flow of mixture on the surface of each filament.
It may be advantageous to insert an annular sleeve that is made of an inert material into components of the feedstock, not fluidtight, that work with the chamber at each orifice and each filament to carry out the wetting of the filament and the suitable flowing of the feedstock onto the filament.
The feedstock flow rate that flows onto the filaments is thus adjusted in combination with the height of the liquid in the chamber, which itself is controlled by a level probe or a measurement of hydrostatic pressure.
The number of filaments is determined based on the desired exchange surface.
The interaxial distance between two filaments can be equal to at least twice the diameter of the filament.
The flow of the fluids that are present is thus promoted, and bringing the latter into contact can be improved by promoting turbulence, at filaments by known suitable means, as a surface treatment for the filaments.
To facilitate the descent of the crystals, it may be advantageous to connect vibration means, ultrasonic, for, example, to means for attaching filaments.
The invention also relates to a process for using the crystallization device for separation of, for example, paraxylene from a mixture of hydrocarbons that contains aromatic isomers with 8 carbon atoms.
This feedstock can be an isomer mixture that corresponds to the composition of a C8 effluent fraction for catalytic reforming or isomerization of xylenes which will require crystallization at very low temperature (xe2x88x9270xc2x0 C., for example), if the maximum yield of paraxylene is desired.
The crystallization feedstock can also be a feedstock that contains at least 60% paraxylene and that results from enrichment by adsorption on a molecular sieve in a simulated fluid bed or coming from a selective dismutation of toluene.
In a more detailed manner, the process for using the crystallization device includes the following stages:
preferably approximately isentropic expansion of a gas that is compressed to an initial pressure and is pre-cooled to a suitable temperature T is carried out, whereby the gas is selected from the group that is formed by air, hydrogen, nitrogen and helium, so that the gas has a temperature that is at least 10xc2x0 C., and preferably 20 to 50xc2x0 C., lower than the temperature that is desired upon its intake into the crystallization zone, defined based on the purity of the hydrocarbon feedstock and the yield of the crystallization,
the expanded gas and cold are introduced at at least one point in the lower part of the crystallization zone,
the hydrocarbon feedstock is adequately introduced into the upper part of the crystallization zone,
the cold countercurrent gas is brought into direct contact with the hydrocarbon feedstock, a suspension of paraxylene crystals is recovered in a mother liquor in the lower part of the crystallization zone, and hot gas is recovered in the upper part of the crystallization zone,
the hot gas is recompressed at a suitable pressure, approximately equal to the initial pressure,
it is cooled at least in part to temperature T,
the crystals of the mother liquor are separated, and the paraxylene crystals are recovered.
Quite obviously, the recovered crystals are washed and purified according to known techniques, for example, in a countercurrent washing column.
This washing stage can be preceded by a stage where the temperature of the crystals that are obtained rises; this makes it possible to homogenize the size of the crystals and to make the crystallization device and purification device operate at their optimum temperatures, which are very different, as is described in Patent Application WO 96/22262.
The feedstock can be introduced by spraying it into the upper part of the crystallization zone, in the form of droplets of 50 to 500 micrometers. As has been stated, the crystallization zone can contain a large number of filaments. The feedstock can also be introduced at a controlled rate, in the form of a film on said filaments with calibrated orifices on the lower wall of the chamber through which these filaments pass.
According to a characteristic of the process, the hot recompressed gas can be cooled to a temperature that is usually between xe2x88x9230 and +70xc2x0 C., advantageously between 20 and 50xc2x0 C., and it can then be expanded at a ratio of 1.5 to 5 relative to the initial pressure, which is the recompression pressure.
The temperature level before the expansion of the gas and the drop in pressure of a gas are determined in relation to the desired cold temperature level in the crystallizer.
According to another characteristic, it is possible to expand the gas at a pressure of 1 at 15 bar and preferably 4 to 8 bar.